Many writers have proposed solar sailing as an energy-free way to explore the Galaxy after the concept of photons was developed early in the twentieth century. The realization that photons, although massless, possessed momentum came from Einsteinís 1905 paper on the photoelectric effect, leading to his Nobel Prize. Apparently, the first references for using solar sails to explore the Universe were made by the Russians in the 1920ís.

The term "solar sail" is occasionally misinterpreted to think that propulsion is derived from the solar wind. This is understandable given our history of marine sailing by atmospheric winds. In fact, propulsion derived from the solar wind is several orders of magnitude less than that from solar radiation. A quantitative comparison between these effects will be discussed below. Another factor that has to be considered is the effect of the Sunís gravity. However light sails may be, they are attracted to the Sun gravitationally, an important factor in sail designs.

Photons have no mass, and travel at the speed of light c, about 300,000 km/sec or 186,000 miles/sec. The energy of photons, E is inversely related to their wavelength, small wavelength photons like X-rays being very energetic while long wavelength photons like radio waves less energetic.

Momentum, p has the classical definition of (mass) x (velocity). For photons however, the momentum imparted to a solar sail would be when the photon is totally absorbed (black body) and when the photon is totally reflected (perfect mirror). The force on a solar sail depends on the reflectivity of the sail, and the solar energy it encounters:

where

F is the force on a of sail in Newtons, N

I is the solar intensity in , about 1,400 at one astronomical unit

k is 0 for a totally absorbing sail, and 1 for a totally reflective sail.

c is the speed of light,

So for a totally reflective sail, the force on a square meter of sail would be about or about of pound force. This is obviously a very small amount of force. However, for huge sails, say 1 kilometer square, the force would increase to 9.3 N. If the combined mass of this large sail and the payload were 10,000 kg, the acceleration would be or 0.93. This is a small acceleration. However, in a 24-hour day, the ship would gain a velocity of or about 180 mph. Although solar intensity drops off as distance from the Sun increases, the ship would rapidly increase speed on an energy-free basis.

Solar wind is an extension to the Sunís corona. It typically has velocities of 300 Ė 800 km/s. Plasma densities range from 1 Ė 10 ions/cc. The ions are about 95% , 4% and 1% heavier components. Assuming an average velocity of 400 km/s () and a plasma density of 3/cc (), we can calculate the solar wind force on a sail of a square meter. Weíll also assume the average mass of each ion is 25% greater than the of , or .

Note that the ratio of the photonic force to that from solar wind is over 9000:1. However, extreme solar wind conditions would create greater force than our example, but rarely would it approach that of photons.

There is another consideration relating to solar sails: the negative effects of solar gravitation. We can calculate the equivalence between gravitational pull and photonic repulsion. Letís assume the best case, the sails are totally reflective:

Newtons

Newtons

setting

or 1.58 grams

So, for a 1-meter square sail with a mass of 1.58 grams, the photonic repulsion is offset by solar gravitational attraction for a sail stationary with respect to the Sun. Current sail masses are about 10 grams per square meter. Obviously, it's necessary to create sails with improved mass ratios to propel a significant payload. However, for sails placed in Earth's heliocentric orbit, even current mass ratios can be potentially useful for small payloads.

There are no firm plans to utilize solar sailing for near-Sun missions. The Planetary Society raised funds for the Cosmos 1's 600 square meter area solar sail, but the Russian rocket failed, therefore, the sail never reached space. Interest in future utilization of this technology has been expressed by NASA, Japan, Russia and Europe. Photonic pressure is planned as a stabilizing influence for the James Webb Space Telescope. The JWST is expected to be launched in 2013. It will be located at the unstable L2 Lagrangian point, and will have a large sunshield to cool the telescope's IR optics. The sunshield, acting like a solar sail, will also have the role of helping to stabilize JWST in its L2 location.